Scribophile looks interesting, but I would actually suggest
Book Country. It's a critique/discussion community for genre writers, set up by Penguin, but completely separate from them. The process there is that to post your work, you review/critique 3 pieces, and then your work will be public. You can post as much or as little as you want, and different people can view different amounts--nonmembers can see about 3000 words, members can see about 10,000, I think, and I believe contacts can see all of it. Plus, when they were setting up the site, they spoke with agents, editors, etc, to make sure posting wouldn't change first publishing rights or chances of getting picked up, or anything.
They will not allow anyone under 18, though, but on twitter, they've linked to
http://writeonteens.com/ for anyone under.
Oh, one of the main people behind it is Colleen Lindsay, who used to be an agent and has worked for several of the publishers.
The big thing I consider a good reason for going there--two authors have been picked up--Kerry Schafer and Michael R Underwood--both with at least 2 books contracted. Underwood's book, Geekomancy, came out from Pocket Star this past Tuesday so it could be hyped at SDCC.
Which reminds me, I should review some stuff so I can start putting up my draft.